A system for obtaining a signal which represents the surface of the roadway is disclosed by DE 41 33 238.5. The signal which is acquired in this way is fed here to an anti-lock brake system and/or traction control system in order to adapt these systems to the state of the roadway, which can take place by influencing controller parameters, e.g. coefficients or threshold values or set values. The degree of unevenness can be divided e.g. into eight classes, each of which brings about a specific adaptation.
If only the degree of unevenness is used, it is necessary to make a compromise between the speed of adaptation and the smoothness of the signal, which compromise may not be satisfactory. If the frequency of the filter is selected to be too low, the signal is very flat; however, sudden changes in the roadway can only be taken into account over a relatively long time frame. In contrast, an excessively high cutoff frequency leads to multiple jumps at the transitions between the classes.